msm: lpm: Add support for registering lpm clock gating mode

An SPM is able to support Clock Gating along with the other low power
modes. Add support for parsing clock gating lpm mode in the lpm driver.

Change-Id: Idac6f7d415cfbe9490aef5c7933d72626e15acae
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>